Active, or smart, materials are being used increasingly in a variety of industries. Active materials can provide benefits in cost savings over optional apparatus for performing like functions, space, or packaging, savings, and savings of resources such as computer-processing or other system-control resources. The materials can also be referred to as transformable materials because they transform, or change state, when exposed to a specific stimulus, as described further below.
In some cases, active materials allow longer system life, faster performance, smoother actuation, increased reliability, and/or more-accurate performance of the sub-system in which the material is used as compared to optional apparatus.
Active materials are being used in industries including transportation, such as in automotive, aerospace, and marine vehicles. Uses are not limited to transport vehicles, though. Active materials can be used in most any system requiring selective actuation of one or more mechanical components.
An active material can be described also as phase-change material because it performs work by changing its phase in response to being exposed to a specific stimulus, such as heat, electric current, lack of heat (e.g., cold), and radiation.
A popular active material is a shape memory alloy, or SMA. Other exemplary active materials include electroactive polymers (EAPs), piezoelectric materials, magnetostrictive materials, and electrorestrictive materials.
Shape-memory alloy is the generic name given to alloys that exhibit the relatively unusual property of having a strain memory, which can be induced by an input, e.g., a mechanical or thermal input. This unusual property is characterized primarily by two thermo-mechanical responses known as the Shape-Memory Effect (SME) and Superelasticity.
Exemplary alloys include copper alloys (CuAlZn), nickel-titanium-based alloys, such as near-equiatomic NiTi, known as Nitinol, and ternary alloys such as NiTiCu and NiTiNb. A particular exemplary allow includes NiTi-based SMAs. NiTi-based SMAs one or the best, if not the best memory properties—i.e., readily returnable to a default shape, of all the known polycrystalline SMAs. The NiTi family of alloys can withstand large stresses and can recover strains near 8% for low cycle use or up to about 2.5% for high cycle use. The strain recovery capability can enable the design of SMA-actuation devices in apparatuses requiring the selective transfer of torque from a torque generating device to each of a plurality of output shafts.
In an Austenite, or parent phase of an SMA, the SMA is stable at temperatures above a characteristic temperature referred to as the Austenite finish (Af) temperature. At temperatures below a Martensite finish (Mf) temperature, the SMA exists in a lower-modulus phase known as Martensite. The unusual thermo-mechanical response of SMAs is attributed to reversible, solid-state, thermo-elastic transformations between the Austenite and Martensite phases.
In some uses, whichever active material used, accurate position control is not needed. If the active material only need flip a switch in response to a pre-determined stimulus, then fine control of the material is not needed. Rather, it is only necessary to stimulate the material sufficiently to actuate the switch, and then allow the material to return to its pre-actuated state.
In many uses, though, fine position control of the material is needed. As an example, many modern vehicles allow users to create personal seat-position settings specifying preferred positioning of seat components including lumbar, seat back, seat base, etc. If the user requests the pre-stored personal seat position, the system needs to move the applicable seat components accurately as needed. For systems using a piece of active material to move one or more of the seat components, the material needs to be controlled carefully in order to accurately effect the desired position of the dependent seat component(s).
To date, a primary means of controlling SMA position involves using an SMA-position sensor. An example sensor is a linear variable differential transformer (LVDT), also sometimes referred to as a differential transformer. An LVDT is a type of electrical transformer used for measuring linear displacement, e.g., position of an item, such as a wire.
Using such sensors has drawbacks including an increase in required space, or packaging, an increase in system cost, an increase in required resources such as computer-processing, slower system performance, and a possible decrease in system robustness or reliability. As can be seen, many of these shortcomings counter corresponding benefits, mentioned above, of using active materials in the first place.
There is a need for systems and methods configured to accurately determine and control position of active-material (e.g., SMA) actuators without using position sensors.